What I do:
To heal sunburn: Gently apply antibiotic cream to the bad sores or some sort of delicate healing cream. Just be sure to not get it in the nose.
To prevent sunburn: I tried the horse sunscreen but it never worked the way I wanted it to. Now I use a water/sweat proof, 40-50ish spf, human sunscreen. Rubbed in well. Then I apply a layer of zinc oxide. For the zinc, I use on of those cosmetic flat cotton rounds to apply it. It's really nasty when you get it on your hands and end up walking around smelling like a baby's bum all day. Lastly, for really sensitive horses, I use a long nose fly mask. Works wonders!
If I remember correctly...
The normal sunscreen is more of a chemical absorber and the Zinc Oxide is more of a barrier.
Hope this helps!